The quality of edges around areas of paint can vary from hard and sharp to soft and diffused, or fading away to nothing. Basically, to create a hard edge paint has to be applied to dry paper, whilst for a soft edge, to damp paper - the damper the paper, the softer and more diffused the edges will be. It is important that each object has the correct type and texture of the edge, or silhouette so that the element is recognisable.
Furthermore, a hard edge suggests space behind the object since there is a line of separation eg rocks, rock faces. Clouds possess soft edges where blending has taken place but also some hard edges will exist giving added interest in the sky.